Medical implants with increased hydrophilicity

ABSTRACT

Devices such as orthopedic implants are composed of a thermoplastic resin such as polyaryletheretherketone (PEEK), and include a ceramic species, such as a zeolite, to enhance its hydrophilic properties. The ceramic species can be a surface coating, can be incorporated or embedded into the thermoplastic resin, or can be both a surface coating and incorporated or embedded into the resin. In certain embodiments, the ceramic species is zeolite that is incorporated into the device, especially at the exposed surface of the device, and is devoid of antimicrobial metal ions. The device is introduced into the body surgically.

This application is a Continuation of U.S. patent application Ser. No. 13/696,346 filed Nov. 27, 2012, the disclosure of which is incorporated herein by reference, which is a 371 of PCT/US2011/035468 filed May 6, 2011, which claims priority of U.S. Provisional Application Ser. No. 61/332,403 filed May 7, 2010, the disclosure of which is incorporated herein by reference.

BACKGROUND

Implantable medical devices are implanted into the body for various reasons including orthopedic applications (e.g., hip replacement, spinal procedures, knee replacement, bone fracture repair, etc). In view of the structural integrity required by such devices, materials of fabrication are limited and generally consist of metal, plastic and composites.

The benefits derived from these devices are often offset by infection which in some cases can lead to sepsis and death. The most common organisms causing infections are Staphylococcus epidermidis and Staphylococcus aureus. Staphylococcus epidermidis is a major component of the normal bacterial flora of human skin and mucous membranes. It is a common pathogen that often colonizes patients in hospital settings who have surgical implants due to the microbes' ability to adhere to medical devices and form a biofilm. Additionally, methicillin-resistant Staphylococcus aureus (MRSA) is a type of staphylococcus bacteria that is resistant to many antibiotics is therefore of particular concern. Other gram-positive bacteria, gram-negative bacteria and fungal organisms also are causative organisms that may be problematic.

As microorganisms come in close proximity to the surface of the medical device, they will either be attracted or repelled by it depending on the sum of the different non-specific interactions. In biological systems, hydrophobic/hydrophilic interactions play an important role in the pathogenesis of a wide range of microbial infections.

Many bacteria can form multicellular coatings, or biofilms, on bioengineered implants. Biofilms facilitate the proliferation and transmission of microorganisms by providing a stable and protective environment for their growth. These biofilms may often result in a broad systemic infection.

In many instances, when implants are seeded by organisms which are protected by tenacious biofilms, the implant must be removed and the patient must be treated with a prolonged course of one or more antibiotics in an effort to cure the infection, after which time a new implant is then reimplanted. This process not only subjects the patient to additional trauma and pain but is also extremely expensive.

Not surprising, a great deal of research has been devoted toward preventing the colonization of the surfaces of orthopedic implants by bacterial and fungal organisms with the use of antimicrobial agents such as antibiotics which may be bound to the surface of these devices.

Thermoplastic resins including polyetherketoneketone (PEKK) and polyetheretherketone (PEEK) have been found to be a useful material for these implants. PEEK is particularly suitable because its modulus of elasticity closely matches that of bone. However, PEEK is a hydrophobic material and bacteria tend to adhere easily to these types of surfaces. It is also an organic material which does not carry significant surface charges. Consequently, it may be desirable to develop a medical implant composed of one or more thermoplastic resins that has reduced hydrophobic properties, and/or that has a net negative charge, particularly at an exposed surface when implanted.

SUMMARY

The shortcomings of the prior art have been overcome by the embodiments disclosed herein, which relate to devices, such as structural orthobiologic materials, particularly intracorporeal devices such as surgical implants, more particularly orthopedic implants, even more particularly spinal implants. In certain embodiments, the device is osteoconductive and is comprised of a thermoplastic resin such as polyaryletheretherketone (PEEK) or polyetherketoneketone (PEKK), and includes a ceramic species, such as a zeolite, to add hydrophilicity and/or a negative charge to the resin. The ceramic species is devoid of antimicrobial metal ions such as silver, copper, zinc, mercury, tin, lead, gold, bismuth, cadmium, chromium and thallium ions. The ceramic species can be a surface coating, can be incorporated or embedded into the thermoplastic resin, or can be both a surface coating and incorporated or embedded into the resin.

In certain embodiments, the ceramic species is zeolite that is incorporated into the device, especially at the exposed surface of the device. The device is introduced into the body surgically. Radio opacity when viewed under X-ray is retained.

DETAILED DESCRIPTION

Embodiments disclosed herein relate to the use of ceramics in combination with medical implants comprising thermoplastic resins such as PEEK, PEKK or the like to alter the hydrophobicity of the polymer and impart a negative charge to the polymer such as at an exposed surface of the polymer, in order to minimize or eliminate biofilm formation, and/or to disrupt the integrity of the biofilm and thus its ability to protect bacteria.

Although the present inventors do not intend to be bound to any particular theory of operation, it is believed that biofilms develop because bacteria attach to the implant. The hydrophobic properties of the implant (now with attached bacteria) prevent antimicrobials from attacking the biofilm containing bacteria. The PEEK/zeolite combination with ionic properties, increases the ability of antimicrobial moieties to permeate in and kill the bacterial pathogen rather than be repelled by the hydrophobic surface properties of naked PEEK.

All bacteria adhere better to hydrophobic surfaces, and it also may be more difficult to detach bacteria from hydrophobic surfaces.

The first stage of bone formation is protein adsorption. Typically, proteins that are most important (i.e., RGD peptide) for bone forming cells attach to surfaces that are negatively charged. Additionally, the charged surface allows the proteins to attach in the correct conformation, leading to the attachment of an optimal number of proteins. The Second stage of bone formation is the attachment of pre-osteoblast cells to the adsorped proteins. These cells then form mature osteoblasts, spread phyllopodia and start the osteoblast maturation/proliferation process. Mature osteoblasts produce ECM (extra cellular matrix) which in combination with the cells, mineralizes into woven bone. PEEK is highly hydrophobic which prevents protein adsorption without which the bone forming process cannot start. By incorporating zeolite into the PEEK structure, a negatively charged surface is created which has the potential to initiate the protein adsorption process.

In addition, tissue does not adhere well to pure PEEK; the tissue simply grows up against the PEEK and forms an interface with minimal adhesion between the two materials. This interface provides an area which is very susceptible to formation of bacterial biofilms, even from systemic bacteria which arrive long after surgery. Since the hydrophilic nature and/or charges on the surface of composite of PEEK and ceramics such as zeolite will foster the growth and adhesion of fibroblasts and osteoblasts that interact with developing tissue, the interface will disappear soon after surgery and the potential for biofilm there will be greatly reduced or even eliminated.

In certain embodiments, the device is configured for use in spinal fusion (arthrodesis) which is often employed to stabilize an unstable spinal column due to structural deformity, trauma, degeneration, etc. Fusion is a surgical technique in which one or more vertebrae of the spine are united together (“fused”) to reduce or eliminate relative motion between them or to fix the spatial relationship between them. Spinal fusions include posterolateral fusion, posterior lumbar interbody fusion, anterior lumbar interbody fusion, anterior/posterior spinal fusion, cervical fusion, thoracic fusion and interlaminar fusion. In certain embodiments, the devices are for insertion in an intervertebral space between adjacent vertebrae. In certain embodiments, a fusion site is identified between adjacent vertebrae and a bone graft is implanted at said site. In certain embodiments, the implant is a spinal interbody cage, including cages comprising titanium, carbon fibers, biocompatible materials such as polyetheretherketone (PEEK), polyetherketoneketone (PEKK), or other synthetic substances. In certain embodiments, zeolite particles are incorporated into the PEEK interbody cage. In certain embodiments, the cage is loaded with osteoconductive and/or osteoinductive agents to promote fusion. Preferably, ceramic particles are incorporated into the resin such that a negative charge is imparted to an exposed surface of the resin. The term “exposed surface” is intended to include one or more surfaces of an implantable device that when implanted, is exposed to or in contact with body tissue and/or fluids.

Either natural zeolites or synthetic zeolites can be used to make the zeolites used in the embodiments disclosed herein. “Zeolite” is an aluminosilicate having a three dimensional skeletal structure that is represented by the formula: XM_(2/n)O.Al₂O₃.YsiO₂.ZH₂O, wherein M represents an ion-exchangeable ion, generally a monovalent or divalent metal ion, n represents the atomic valency of the (metal) ion, X and Y represent coefficients of metal oxide and silica respectively, and Z represents the number of water of crystallization. Examples of such zeolites include A-type zeolites, X-type zeolites, Y-type zeolites, T-type zeolites, high-silica zeolites, sodalite, mordenite, analcite, clinoptilolite, chabazite and erionite.

Zeolites can be obtained in master batches of pellets of low density polyethylene, polypropylene, ultra high molecular weight polyethylene or polystyrene, containing suitable amounts of zeolite particles, usually 20 wt. % of zeolite particles. When provided in this form, the pellets of resin containing the zeolite particles can be easily mixed with resins used to make the implants or used to make coatings to be applied to the implants, as set forth in U.S. Pat. No. 6,582,715, the disclosure of which is hereby incorporated by reference. Typical amounts of zeolite particles incorporated in an implant resin range from 0.01 to 10 wt. %, more preferably 0.01 to 8.0 wt. %, most preferably 0.1 to 5.0 wt. %. The method used to coat or otherwise incorporate the ceramic in to the resin is not particularly limited, and can include spraying, painting or dipping. When compounded into PEEK, for example, the PEEK should be protected from sources of moisture and contamination. The compounding can be carried out by blending. The ceramic species can be a surface coating, can be incorporated or embedded into the thermoplastic resin, or can be both a surface coating and incorporated or embedded into the resin.

Other suitable resins include low density polyethylene, polypropylene, ultra high molecular weight polyethylene or polystyrene, polyvinyl chloride, ABS resins, silicones, rubber, and mixtures thereof. These can be formulated to contain suitable amounts of zeolite particles, usually about 20 wt. %. An UHMWPE is preferred for the implant devices.

The masterbatch is a concentrated mixture of pigments and/or additives (e.g., zeolite powder) encapsulated during a heat process into a carrier resin which is then cooled and cut into a granular shape. Using a masterbatch allows the processor to introduce additives to raw polymer (let down resin) economically and simply during the plastics manufacturing process.

The zeolite incorporated into the resin and implanted is devoid of antimicrobial metal ions.

Under conditions of high temperature and high shear, the zeolite is incorporated into the resin, such as by mixing doped metal zeolites into molten PEEK (melting point between 300 and 400° C.), followed by molding and processing of the composite blend.

EXAMPLE

About 5% by weight of the zeolite powder is mixed thoroughly with the powdered or prilled PEEK. The mixture is brought up to temperature and processed at 400° C. using high shear. The zeolite and PEEK must be dry before processing in order to minimize decomposition and void formation in the product.

The material can be formed into prills for further processing, cast into blocks, extruded into rods or injection molded into the final desired shapes.

The block and rod materials can be machined into shapes which are suitable for use as orthopedic implants or other designs where antimicrobial PEEK finds application. Implants can be designed to provide enhanced surface area by having grooves cut in the surfaces or by producing products with holes in the body of the pieces. Surface area can be further enhanced by sanding or abrasive blasting of the surfaces. 

What is claimed is:
 1. A method of initiating bone formation, comprising implanting in said patient a device comprising a thermoplastic resin having aluminosilicate particles incorporated therein, said aluminosilicate particles being devoid of antimicrobial metal ions and being present in said coating in an amount sufficient to initiate protein adsorption on said implant.
 2. The method of claim 1, wherein said thermoplastic resin comprises PEEK.
 3. The method of claim 1, wherein said implant is an interbody spinal cage.
 4. The method of claim 1, wherein said aluminosilicate is represented by the formula XM_(2/n)O.Al₂O₃.YsiO₂.ZH₂O wherein M represents an ion-exchangeable ion, n represents the atomic valency of the (metal) ion, X and Y represent coefficients of metal oxide and silica respectively, and Z represents the number of water of crystallization. 